The invention relates to a method for diagnosing an exhaust gas recycling system of an internal combustion engine according to the preamble of patent claim 1.
Exhaust gas recycling systems are used to reduce the nitrogen oxide content in the exhaust gas of an internal combustion engine. A portion of the exhaust gas is recycled from the exhaust gas tract into the intake tract, i.e. into the collector of the intake manifold, via an exhaust gas recycling line. An exhaust gas recycling valve which is arranged in the exhaust gas recycling line is actuated by means of electrical signals of a control device, and the mass of the recycled exhaust gas is thus set. As the exhaust gas of the internal combustion engine is, in its essential components, an inert gas, it is possible to reduce the peak temperature of the combustion by adding exhaust gas to the sucked-in internal combustion air, and thus to reduce the emission of nitrogen oxides. The mass of the recycled exhaust gas in relationship to the sum of the mass of the fresh gas (intake air) and the mass of the recycled exhaust gas is generally referred to as exhaust gas recycling rate.
It is necessary to monitor the controllability of and the possibility of closing such an exhaust gas recycling valve in order to ensure that it is functioning correctly. In order to maintain the exhaust gas limiting values which are required by legislators, it is necessary to equip the vehicles with the diagnostic devices which permit the malfunctions of sensors and systems which are connected to the controller or activation of exhaust-gas-related parts to be registered, and corresponding fault messages to be output. Particularly the checking of all the components of the air path is of particular interest for the fulfillment of legal guidelines for the onboard diagnosis (OBD) of motor vehicles.
In particular the exhaust gas recycling valve must be able to close completely as otherwise the prescribed exhaust gas limiting values may be exceeded when the internal combustion engine is operating with a permanently opened exhaust gas recycling valve.
EP 0 635 629 A1 describes a method for diagnosing an exhaust gas recycling system of an internal combustion engine in which the exhaust gas recycling valve is actuated during the stable idling mode for a specific diagnostic time with a diagnostic pulse duty factor and during this time the reaction to the rotational speed engine operating parameter is evaluated. The difference between the rotational speed which occurs during the stable idling mode of the internal combustion engine before the exhaust gas recycling valve is actuated and that rotational speed which occurs when the system is actuated with a diagnostic pulse duty factor is formed. If the difference exceeds a predefined diagnostic limiting value, it is determined that the exhaust gas recycling system is intact, and otherwise a fault message is output.
DE 195 27 030 A1 discloses an abnormality-registering method and a device for an exhaust gas recycling control system of an internal combustion engine. The abnormality is caused here by an operative lag of a diaphragm which forms an exhaust gas recycling valve. The exhaust gas recycling valve is controlled as a function of the engine operating information including an intake distributor pressure. A condition which is to be fulfilled for the abnormality decision to be activated is registered. When the abnormality decision condition or activation condition is registered, the exhaust gas recycling valve is forcibly opened or closed. A possible abnormality of the exhaust gas recycling valve is decided on the basis of the difference between the intake distributor pressure during forced opening and that during forced closing of the exhaust gas recycling valve.
The object of the invention is to specify a method with which the operational capability of the exhaust gas recycling system of the internal combustion engine can easily be checked without additional sensor equipment.
This object is achieved according to the invention by the following:
A method for diagnosing the operational capability of an exhaust gas recycling system of an internal combustion engine having cylinders, said system comprising an exhaust gas recycling line which connects an exhaust gas tract of the internal combustion engine to an intake tract, having a throttle valve of the internal combustion engine so that exhaust gas can be recycled into the intake tract; and an exhaust gas recycling valve arranged in the exhaust gas recycling line to set the flow passage of the exhaust gas recycling line, said method comprising the following steps when a fuel supply to the cylinders is switched off and when a vehicle powered by said engine is in a stationary state:
in a first step, the throttle valve and the exhaust gas recycling valve are closed,
in a second step, after expiry of a predefined time period, checking to determine whether a air mass flow rate meter signal lies below a predefined threshold value,
in a third step, if said value drops below the threshold value, checking to determine whether compression has occurred in the cylinders of the internal combustion engine, and, assessing the desirability of
closing the exhaust gas recycling valve as a function of the check.
If the throttle valve and exhaust gas recycling valve are closed when the internal combustion is switched off, the controllability of and possibility of closing the exhaust gas recycling valve can be monitored using an air mass flow rate sensor and a rotational speed sensor which are present in any case. In this way, the operational capability of the exhaust gas recycling valve can be diagnosed during the otherwise fault-free operation of the vehicle and when there are defined output operating parameters by reference to the profile of the signals of these two sensors, using applicable time delays and threshold values.
The invention is distinguished by the fact that, in order to diagnose the exhaust gas recycling valve when the internal combustion engine is switched off, the throttle valve and the exhaust gas recycling valve are closed and a check is made by evaluating the rotational speed within a segment to determine whether a specific degree of compression can still be detected in the cylinders of the internal combustion engine. If a rotational speed difference above a threshold value is present, an incompletely closing exhaust gas recycling valve is detected, assuming complete closure of the throttle valve.
The method has in particular the advantage that no additional sensor equipment is necessary to check the controllability of and possibility of closing the exhaust gas recycling valve.